The observed physiological response to hormone treatment is the coordinate regulation of tissue-specific gene networks. The effects of steroid hormones are mediated by intracellular receptor proteins. Hormone receptor complexes are tightly associated with the nucleus and have high affinities for specific DNA sequences found within and near hormone-responsive genes. Glucocorticoid receptors (GR) have been extensively characterized at the biochemical and genetic level; this analysis has been facilitated by the recent isolation of GR coding sequences. The site of GR action on the DNA has also been examined in detail; the GR binds specifically in vitro to the same sequence which acts in vivo as a hormone-inducible transcriptional enhancer. One example of a cell specific hormone response is the GR-dependent induction of cytolysis in two murine thymoma cell lines (WEHI7) and S49). The molecular basis for the observed cell death in these cells is not known, but is most likely regulated at the level of transcription by the GR. Since glucocorticoids are useful in the treatment of several human lymphatic cancers, it has been suggested that the hormoneinduced cell death of WEHI7 and S49 cells may be a good model system for the study of clinical responses to steroid therapy, such as tumor regression and cytolysis of leukemic cells. We propose to use several molecular biological approaches to identify the mechanism of hormone-regulated cytolysis. First, we will determine what GR activities are required for cytolysis by isolating and characterizing GR coding sequences (cDNA and genomic) from a unique class of steroid resistant mutants which express a form of the GR that binds hormone and DNA, but is apparently defective in its ability to modulate transcription. Second, we will isolate genes required for lysis using the techniques of subtraction cDNA cloning and retrovirus-mediated transposon tagging. The long term goal of this study is to delineate tissue-specific glucocorticoid gene networks using molecular genetics and to understand how GR-dependent gene expression is coordinately and developmentally regulated.